eg. a quick search found a Ham radio group in SD: https://www.facebook.com/groups/856599821517281.
More Ham radio groups in San Diego (you can do a similar search for retro computing): https://www.facebook.com/search/groups?q=ham%20radio&filters...
Meetup also has some retro computing groups, though I don't see one in San Diego (why not start one?): https://www.meetup.com/topics/retro-computers/
(Caveat: I work on FB Groups)